Wafer temperature is an important process parameter in many semiconductor fabrication processes including rapid thermal processing ("RTP"). Numerous semiconductor device thermal fabrication processes employ noninvasive temperature sensors which require calibration in reference to reliable and repeatable standards. An important group of device fabrication processes is rapid thermal processing. Most of the RTP reactors employ non-contact single-point or multi-point pyrometry for wafer temperature measurement and control. However, the accuracy and repeatability of RTP temperature measurements by pyrometry depend strongly on the wafer emissivity. In practice, frequent pyrometry sensor calibrations are required in order to obtain acceptable process repeatability and uniformity. These frequent cross-calibrations are usually performed by placing separate standard calibration wafers with bonded thermocouples (herein after referred to as "TC-bonded wafers") in the process chamber. However, these commercially available calibration wafers have numerous drawbacks.
First, TC-bonded wafers are fabricated using a long and expensive manual process. The fabrication process includes drilling holes in the silicon wafer to place the thermocouple junction. Then a ceramic glue is used to secure the thermocouple junctions. As a result, the TC-bonded wafers are rather expensive. In addition, the cost to fabricate these wafers dramatically increases with the number of bonded thermocouples required on a wafer. Calibration wafers with multiple sensing elements are required to monitor and adjust process uniformity.
Second, wafers with bonded thermocouples have limited lifetime for high-temperature calibration applications. This limited lifetime problem is even worse when the calibration procedures are performed in reactive ambients such as oxygen. As a result, a TC-bonded calibration wafer may survive only for a few, typically less than 20, calibration runs before the thermocouple junction degrades. This makes the effective temperature calibration cost relatively high.
Third, bonded thermocouples result in thermal loading of the wafer due to the thermal mass of the thermocouple junctions and the thermocouple wires connected to the junction. This can result in disturbance of the wafer temperature at the thermocouple junction locations.
Fourth, TC-bonded wafers with multiple thermocouples contain wires distributed over the wafer. These wires typically extend over the entire wafer surface, when multiple thermocouples are utilized, and can cause disturbance of wafer temperature distribution and make wafer handling difficult.
It is an object of the present invention to provide a means for calibrating thermally activated fabrication processes that are reliable, cost-effective, and accurate. In conjunction with these objectives, it is also an objective of the present invention to provide a temperature calibration device that has an extended lifetime, as compared to TC-bonded wafers. Moreover, another object of this invention is to provide a temperature calibration device with as many temperature calibration elements as required. Further objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.